Patent Cooperation Treaty Publication WO 96/11047 describes apparatus for the removal of particulate material suspended in a gas stream, in which rotary motion is generated in the stream which is then passed to an expansion chamber where the gases are decelerated to enable particulates to fall out of suspension and to be collected in a collection chamber. It has been found that, whereas such apparatus deals effectively with large and medium-sized particles, small particles tend to remain entrained in the gas stream and are exhausted to the atmosphere. Clearly, where the apparatus is incorporated in a suction cleaner for indoor use, the result is that the small particulate matter tends to be recycled in the room being cleaned and, having left the apparatus in the exhaust stream, it eventually settles as dust on the furniture or the floor.